Including Discussion of Prior Art
It is well known, that in compression type refrigerating and air conditioning systems, that the compressors employed for compressing the vaporous refrigerant employ oil or lubricant to lubricate their internal parts. In the course of drawing in refrigerant vapor, compressing and discharging the vapor to condensers, some of the lubricating oil is entrained with and discharged with the compressed vapor. Though means for removing some of the oil from the discharge stream are sometimes employed, a small quantity of oil always fails to be removed, traversing such means, and is conveyed with the compressed refrigerant through the condenser and cooling coil or evaporator. The oil leaving the evaporator must have a way to return to the compressor. Failure to provide such a way results in accumulation of oil within the refrigerating tubes and pipes and progressive loss of oil from the compressor. It is not uncommon in poorly designed systems for so much oil to be lost from the compressor that insufficient oil is left to properly lubricate and cool the compressor, and the compressor fails.
Wherever the compressor is at the same level or lower than the evaporator, oil flow through the vapor return conduit from evaporator to compressor (suction line) is aided by the velocity of vapor flowing through the suction line and by gravity and the oil returns satisfactorily to compressor. Even when the compressor is located higher than the evaporator, satisfactory oil return can be simply secured by proper sizing of the upflowing suction line (suction riser) to provide adequate vapor velocity to assure oil return.
However, the compressor-overhead situation is severely complicated when there are several evaporators at various levels below the compressor and the evaporators operate on independent schedules so that the refrigerating loads and therefore the gas velocities through the suction riser vary widely.
One of several strategies found in piping manuals are employed now to cope with this situation. One strategy employs so-called dual risers, where a large and a small riser are coupled together at their bottom and an oil trap is employed to stop flow through the large riser, thereby maintaining satisfactory vapor velocities through the small riser to assure oil return. This arrangement works satisfactorily when the range of loads is small, typically 4:1.
Under this parallel condition the vapor velocity in both must be sufficient to cause oil to flow up the risers. Naturally, great precision and engineering skill is required to properly size the risers and traps. Further, where the loads vary widely, over a range of 10 to 1 or more, such dual riser systems fail to work and oil accumulates in the risers and is lost from the compressor/s. This situation is further complicated and worsened where there are multiple compressors which operate under independent control so that even a single small compressor may run while still requiring satisfactory oil return.
A serious draw back of the dual riser arrangement is that the oil trap removes oils that may be needed for compressor lubrication.
When the pressure drop through the small riser becomes so great that it blows out the oil trap, then both risers function together in parallel. A second serious draw back is the likelihood that the mass of oil accumulated in the trap will be carried back to the compressor in a slug when the load suddenly increases, thereby raising the possibility of compressor damage from a mass of incompressible oil entering its cylinders.
A second strategy simply requires that each evaporator have its own suction riser, sized for proper return of oil when the evaporator is operating.
Where loads vary very widely and a unitary riser system is desirable, engineers have employed oil accumulators at the bottom of the risers to collect oil which fails to be returned up the riser at conditions of low load and corresponding low suction vapor velocities. This arrangement requires the use of pressure pumps to force the oil collected in the oil accumulator back to the compressor/s through small pipes provided for the purpose.
The present invention is directed to solving this problem in a simple manner, without oil pumps, without critical pipe sizing all while ensuring proper oil return to the compressor over an extremely wide range of full load to minimum load ratios.